The present application generally relates to compositions of matter and methods for converting waste grease to green diesel fuel.
Environmental awareness and projected increases in the world's energy demand have been the motivation for seeking environmentally friendly, renewable alternative fuels. A large amount of waste cooking oil and grease is produced in the U.S. that can be exploited for liquid biofuel generation. In particular, brown grease, which contains mainly free fatty acids (FFAs), can be a potential inexpensive source for a process to obtain straight chain hydrocarbons in the diesel fuel boiling range (green diesel) via catalytic decarboxylation.
Recently, there has been considerable attention on the development of suitable catalysts for decarboxylation of free fatty acids (FFA). Most early studies focused on Pd-based catalysts, which exhibit high activity and selectivity for the formation of straight chain hydrocarbons with one carbon number less than the source FFA. However, these supported palladium catalysts readily deactivate even in the presence of H2. Although a 3 wt % Pd-SBA-15 catalyst is active at 300° C. under 17 bar of 5 vol % H2 in argon for stearic acid decarboxylation for 5 hours, deactivation is reported due to the formation of unsaturated heptadecene product. A 1 wt % Pd supported on a synthetic mesoporous carbon catalyst shows 23% decrease in the BET specific surface area after decarboxylation of palmitic and stearic acids mixture at 300° C. and 17.5 bar H2/Ar. In all cases, the extensive catalyst deactivation may be attributed to catalyst coking. Catalyst deactivation may be related to the amount of unsaturated products which further led to catalyst coking specifically for Ru/C and Rh/C catalysts after 6 hours of stearic acid decarboxylation. On the other hand, the Pd/C catalyst deactivation may be attributed to the reaction atmosphere and degree of unsaturation of the FFA or to catalyst supports.
Deactivation of a mesoporous silica supported palladium catalyst may occur during FFA decarboxylation due to the loss in total surface area, porosity and accessible palladium surface area. Unlike the previously reported literature claim of coke formation, it is claimed that strongly adsorbed reactants and products cause the deactivation. The stability of 5% Pd/C in fatty acid hydrothermal decarboxylation has been investigated and it is reported that the decarboxylation activity of the catalysts is maintained although metal dispersion is significantly reduced after catalyst reuse. The difference in catalytic behavior of the supported metal particles is attributed to the hydrothermal reaction where the catalyst is exposed to sub-critical water.
An ordered mesoporous silica-carbon catalyst support is synthesized as a novel hybrid material. This nanocomposite support has gained increasing attention for catalysis applications in recent years due to several unique features such as high dispersion of palladium nanoparticles (about 3 nm), high surface area, large and tunable pore structure and excellent stability. These silica-carbon nanocomposites are produced on the basis of a triblock copolymer templating approach which is a time consuming catalyst preparation technique.
The nature of the surface functional groups on the activated carbon support when modified by oxidative treatments is found to be a factor in the catalytic activity of precious metals such as palladium. After introducing such oxygen groups, the surface behavior of carbon changes; therefore their catalytic properties differ. The components of activated carbon are disorganized polyaromatic sheets with reactive corner atoms and adsorbent surface atoms. The precursor that is selected for this study, TEOS, is expected to form the templates that contain —OH groups and bridged O atoms in a Si—O—Si structure on the amorphous silica walls, and these groups play a role for the incorporation of silica into activated carbon.
Environmental, economic, and energy security concerns have been the motivation for seeking environmentally friendly, renewable alternative fuels. The major feedstocks for non-ethanol liquid biofuel production are vegetable oils and animal fats. Waste oils, such as used frying oils and brown grease, are lower-cost lipid feedstocks and currently a potential source for economical production-oriented approaches.
From environmental point of view, a life-cycle analysis of different fuel production routes shows that both biodiesel and green diesel products have much lower total environmental impact scores than petroleum diesel. Herein, biodiesel refers to a mixture of fatty acid methyl esters while green diesel refers to a mixture of hydrocarbons in the diesel boiling range that possesses similar fuel properties as petroleum diesel.
There is a tremendous amount of waste cooking oil and grease, collected from restaurant traps, that may be exploited for fuel use. The total volume of trap grease, or brown grease (BG) produced is ˜3,800 million pounds per year in the U.S. Disposing of brown grease is a costly process. On the other hand, brown grease is known to possess a high energy value of around 12,000 Btu per pound. Furthermore, brown grease is an inexpensive feedstock in comparison with food grade vegetable oils. The primary cost factor of green diesel is determined as the feedstock costs. It is concluded that soybean oil requires a subsidy in order for the new technology to be competitive with the current crude oil refiners. Therefore, substituting soybean oil feedstock with brown grease would have a significant impact on the economics of green diesel technology. However, the high free fatty acid (FFA) content of brown grease (50-100%) can be problematic for biofuel production and there is no proven biofuel production technology for a feedstock having 50-100% FFA content. The presence of FFA in the feedstock of vegetable oils also creates processing problems. When 10 wt. % FFA-90 wt. % triglycerides are used in a hydrotreating process to produce green diesel, the fraction of high molecular weight hydrocarbons products not in the diesel fuel boiling range gradually increased compared to a feedstock containing only triglycerides. This resulted in a loss of diesel yield and reduction in catalyst life.7
Brown grease is comprised of both saturated and unsaturated FFAs. Almost 40% of brown grease is oleic acid (C18:1), which is a monounsaturated fatty acid, and around 70% is total unsaturated fatty acids. Due to its high FFA content (50-100%), BG is potentially a good candidate for a decarboxylation reaction where the oxygen is removed as carbon dioxide, producing green diesel. Currently, hydrodeoxygenation (HDO) is the only proven technology to convert waste oil into green diesel. However, this technique requires high pressure (˜5 MPa) and excess H2 (H2/oil ratio of ˜1000/1) in order to remove oxygen as water, leading to high production costs. In comparison, decarboxylation does not require additional H2 to form hydrocarbons. Although several studies of hydrocarbon production from waste oil and vegetable oil (or refinery oil) mixtures have been reported, no selective decarboxylation of brown grease for the production of diesel fuel hydrocarbons has been demonstrated.
Saturated fatty acids have been successfully converted to hydrocarbons via decarboxylation under inert gas. Screening of heterogeneous catalysts for decarboxylation of stearic acid as the model FFA compound has been performed with different metals (Ni, NiMo, Ru, Pd, PdPt, Pt, Ir, Os, Rh) on different supports (Al2O3, SiO2, Cr2O3, MgO, C) under a helium inert gas atmosphere. A 5% Pd on activated carbon supported catalyst provided the best conversion of stearic acid to C17 “green diesel like” hydrocarbons (mainly n-heptadecane), with 100% conversion of stearic acid and 99% selectivity to total C17 hydrocarbons. The high decarboxylation activity of 5% Pd/C is attributed to the significantly higher specific surface area of activated carbon than the metal oxide supports and the ability of Pd to form Pd/H complex which acts as a catalytic site for decarboxylation.
There has been considerable study of the conversion of unsaturated FFAs to hydrocarbons. However, there is not yet an active and selective catalyst that can handle direct decarboxylation of unsaturated FFAs to hydrocarbons. The best results demonstrated so far are 99% conversion of oleic acid to stearic acid (selectivity (S)=36%), heptadecane (S=26%) and other side products after 6 hours over Pd/C catalyst under Ar—H2 flow, at 300° C. and 2.7 MPa. Because of the competitive adsorption and reaction of active C═C double bonds on the catalyst surface, the decarboxylation yield of total FFAs decreased while yield of the side reactions increased, leading to an increased H2 consumption and a decreased diesel yield.
During the reaction to convert oleic acid to n-paraffins over 5% Pd/C in the presence of 10% H2 and solvent (dodecane) at 1.5 MPa and 300° C., the primary reactions are hydrogenation of C═C double bonds followed by decarboxylation of the resulting stearic acid. However, in the absence of H2, oleic acid decarboxylation is inhibited by adsorbed cis-C═C double bonds in its alkyl chain.
Increases in petroleum prices, projected increases in the world's energy demand and environmental awareness have shifted research efforts to explore alternative fuel technologies. In particular, green diesel which displays similar properties as petroleum diesel and can be used as a drop-in fuel, has drawn great attention. This second generation liquid biofuel can be obtained from triglycerides and fatty acid containing feedstocks such as vegetable oil, animal fat and waste oil/grease. However, converting waste oil/grease, particularly brown grease which possesses 50-100% fatty acid content, into biofuels is more advantageous because it is a waste, inexpensive and non-food competing feedstock. In the U.S. alone, 3800 million pounds of brown grease is generated every year. There has been considerable attention on the production of green diesel from vegetable oil and fat. Most early studies focused on deoxygenation (selectively decarboxylation) of fatty acids in dodecane solvent over Pd-supported catalysts. These studies demonstrated milder reaction conditions and elimination of hydrogen consumption can be possible compared with the current commercial process (hydrotreating). However, these supported palladium catalysts readily deactivate due to the formation of unsaturated heptadecene product leading to catalyst coking, the high unsaturation level of the fatty acids, lack of hydrogen in the reaction atmosphere, decrease in the BET specific surface area, loss in porosity and accessible palladium surface area.
Recently, studies of fatty acid deoxygenation have been conducted in aqueous media under sub- and super-critical water conditions. The advantage of water as the reaction media is not only the use of an environmentally benign solvent in the process but also the avoidance of a water removal step after biomass conversion or triglyceride hydrolysis that generates fatty acids in an aqueous stream. It is shown that both Pd/C and Pt/C catalysts are active for a saturated fatty acid (palmitic acid) decarboxylation with 76% molar yield to pentadecane in subcritical water at 370° C. However, Pt metal dispersion exhibited a significant reduction (from 38.9% to 0.8%) after the reaction. Activated carbon itself can catalyze both saturated and unsaturated fatty acids to produce hydrocarbons in sub- and super-critical water as an alternative to the expensive noble metal catalyst. However, the major product from oleic acid conversion is stearic acid with 24% molar yield while the decarboxylation product yield is only 6% after 3 hours reaction at 370° C.
Pd/C catalyst behaves differently in sub-critical water than in organic solvent for fatty acid decarboxylation.
A decarboxylation study of acetic acid, one of the simplest carboxylic acid, conducted on ZrO2 in super-critical water at 400° C., shows that ZrO2 is an active catalyst for CO2 removal from acetic acid, however, it selectively produces acetone (ketone). Moreover, a structure change of the zirconia catalyst is observed during acetic acid conversion in super-critical water. The conversion of stearic acid in the presence of oxide catalysts (CeO2, Y2O3 and ZrO2) is reported as 30%, 62% and 68%, respectively in super-critical water at 400° C. in 30 minutes. Similar to the acetic acid hydrothermal reaction, stearic acid reaction produced ketone (C17H35OCH3) in addition to hydrocarbons. Structure change of another oxide catalyst is also observed in a solvent free oleic acid decarboxylation reaction in an investigation of the decarboxylation activity of hydrotalcites catalysts with various MgO/Al2O3 ratios in a solvent free atmosphere, that a MgO loading of more than 63% and reaction temperature of 350° C. is needed to obtain deoxygenated hydrocarbon products selectively and with oleic acid conversion more than 98% in 3 hours. More importantly, there is no significant change of the MgO structure in the hydrotalcite catalyst.
Stearic acid thermal decomposition is observed with 50% conversion at 400° C. under Ar atmosphere in 30 minutes while its hydrothermal conversion is 2% in super-critical water under the same reaction conditions. Stearic acid conversion is enhanced by adding NaOH or KOH in super-critical water.
Degree of fatty acid unsaturation on the decarboxylation over Pt/C catalyst in sub-critical water at 330° C. is that unsaturated fatty acids possess much lower heptadecane yield and selectivity than saturated fatty acids (molar yield of more than 80% to heptadecane from stearic acid vs. less than 20% from oleic acid) in 2.5 hours reaction. Because Pt/C catalyst is found to be more active and selective for decarboxylation of palmitic (a saturated) acid compared to Pd/C in sub-critical water, oleic acid decarboxylation over Pt/C catalyst is investigated. However, saturated and unsaturated fatty acids behave differently under hydrothermal reaction conditions.
Conversion of waste oil/grease which mainly contains unsaturated fatty acids may be predicted by investigation of oleic acid (the major component of waste oil) conversion on Pd metal supported catalyst. In order to design a suitable catalyst for conversion of brown grease to green diesel, a systematic study of the model compounds is necessary to understand the reaction pathways in super-critical water. Therefore, the decarboxylation reaction of oleic acid is investigated on various catalysts in super-critical water with the aim of producing hydrocarbons in the diesel range in the absence of H2 and to improve the catalytic decarboxylation activity and selectivity of the carbon supported catalyst.
There is a need to investigate the effect of reaction parameters on the activity and the selectivity of brown grease decarboxylation with minimum H2 consumption over an activated carbon supported palladium catalyst, and to gain a better understanding of the reaction pathways.
In the present work, a new, well-defined and highly efficient Pd/Si—C catalyst is developed for the decarboxylation of FFA. This new nanostructured hybrid catalyst has a well-defined mesoporous structure which allows a better understanding of structure-activity characteristics that are crucial in elucidating the FFA decarboxylation mechanism, unlike an activated carbon supported palladium catalyst. The decarboxylation reaction of oleic acid is investigated over these catalysts with the aim of producing green diesel in the absence of additional H2 under mild reaction conditions, elucidating the effects of the nature of the functional groups on the activity and developing a procedure to maintain high catalytic activity. For all these reasons, there is a need for catalytic processes for the conversion of unwanted brown grease into valuable products such as biofuels.